A typical disk drive employs a rotary actuator motor to move an actuator for positioning the read/write heads over a disk media. Magnetic flux for the rotary actuator motor is typically generated by a magnetic circuit comprising a return plate and a pair of magnets. The return plate is usually comprised of a top plate, a bottom plate, and several standoffs. The standoffs hold the top and bottom plates apart at a fixed distance to form an air gap for receiving an actuator coil coupled to the actuator. Typically, magnets are bonded to the inner walls of both the top and bottom plates. The return plate is usually coupled into the disk drive via a set of bosses formed in the base plate of the disk drive.
The top and bottom plates as well as the standoffs are typically constructed of a magnetic permeable material such as a low carbon steel. The top and bottom plates along with the standoffs form a flux return path for a magnetic field generated by the top and bottom magnets. The magnetic flux within the air gap between the magnets causes induces torque on the actuator according to an electric current flow in the actuator coil.
Typically, the top and bottom plates and the standoffs employ circular clearance holes for receiving the bosses formed in the base plate. The bosses extend through the clearance holes when the return plate is assembled into the base plate. The clearance holes have loose tolerances to compensate for the tolerances in the diameters of the bosses, and the tolerances in the locations of the bosses on the base plate. A screw is usually coupled through the actuator bearing to provide a pivot point for the top and bottom plates.
However, the circular clearance holes employed in prior return plate designs can cause misalignment between the top and bottom plates. The misalignment occurs when the angular position of the top plate around the pivot point differs from the angular position of the bottom plate around the pivot point. The differing angular positions of the top and bottom plates are a consequence of the loose tolerances of the circular clearance holes.
Unfortunately, the misalignment between the top and bottom plates causes a corresponding misalignment between the top and bottom magnets. The magnet misalignment reduces the linearity of actuator torque across the range of actuator movement over the disk media. The reduced linearity of the actuator must be compensated for by the servo control system, thereby increasing disk drive access time and reducing disk drive performance.